Knife sharpener



FIG. 1 is a perspective view of a knife sharpener embodying the new design;

FIG. 2 is a right side elevation view of the knife sharpener, the left side elevation view being a mirror image of the right side;

FIG. 3 is a front elevation view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a rear elevation view thereof; and,

FIG. 6 is a bottom plan view thereof.

The broken lines depict portions of the knife sharpener in which the design is embodied that are not considered part of the claimed design. 

CLAIM The ornamental design for a knife sharpener, as shown and described. 